At the present time almost all sample application, spotting on TLC (thin-layer chromatography) plates is done by means of micro capillary pipets, either by hand or arranged in series in a mechanical device, to increase the efficiency and ease of the spotting operation.
The samples held and delivered by a micro capillary pipet are liquids in which the sample is dissolved and must, for analytical purposes, be generally delivered in quantities of 1 to 2 .mu.l at a time, to keep the area of sample application (spot) small and the sample itself as concentrated as possible. In more elaborate micro capillary spotters a stream of warm air will continuously pass over the area of sample application, or that area will be warmed by an electric heating block, to speed up evaporation of the carrier liquid and dry the samples so that the spotting operation can be speeded up or sample delivery can be made on a continuous basis from micro capillaries with access to a sample reservoir. These latter devices also contain a more complex plumbing arrangement.
For concentrated samples of a few .mu.l, hand spotting with a micro capillary pipet is probably the easiest, most cost efficient and rapid method available. However, as the sample size increases to the 20 to 50 .mu.l range, commonly encountered in drug analysis, and large numbers of such samples must be spotted on a day to day basis, hand spotting becomes a very tedious, lengthy and laborious task. It also demands a great deal of precision on the part of the operator to apply successive increments of a sample to the same location on a plate so as to keep the spot size small. The capillary spotting devices reduce the amount of hand labor and the need for precision on the part of the operator, but still require a great deal of set up time and maintenance to replace the capillaries, fill and clean sample reservoirs, purge and clean plumbing, etc. In addition, these capillary spotters are generally fairly expensive and some of the components such as the glass capillaries, reservoirs, and plumbing are quite fragile and subject to breakage.